<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
    <style>
        div{
            width: 100px;
            height: 100px;
            background: red;
            padding: 20px;
        }
    .a{
        z-index: 5;
    }
    .b{
        margin-top: -100px;
        z-index: 2;
        background: #ccc;
    }
    </style>
</head>
<body>
    <div class="a">111</div>
    <div class="b"></div>
</body>
</html>